Tracer rounds work both ways, die rolls too!

As a GM every time you ask your players to roll to hit you're a dead give away on the enemy strength. In fantasy settings with bows and arrows and close range combat this isn't much of an issue. If the enemy is close enough to attack you he's close enough to be see, unless some magic is in use.

In modern or futuristic combat, this is not the case. You may come under fire from a great distance and have only the general direction to which you can return fire. "You're taking fire from the tree line on the ridge", what does your party do? "We fire back at the attackers.", but wait, what attackers?

Rules like d20 require a target to hit. They're not very good at determining area of effect. So a burst of return fire or a barrage of suppressing fire would be hard to resolve. As a GM you'll probably have to ask the players to roll a certain amount of dice to see if their attacks hit. But that's a dead giveaway on the enemy force's size. Is the enemy just a sniper and his spotter? A platoon? A whole freaking brigade?

You'll probably end up putting a bunch of little icons on Roll20 as well, indicating the enemy combatants on the field. But wait, isn't that what camouflage is for? So they can't be seen and shot at? The game's combat mechanics is making you, the GM, give valuable information the players that their characters would not normally have in real combat situations.

I think opposing die roll mechanics are ideal for this, as they don't rely on a single die roll from the attacker (player). With opposing die rolls the attacker can set the degree of effectiveness of the attack with a single roll and the defender(s) can counter it, one by one. This allows the GM to secretly roll for the opponents, and in doing so hide the enemy force size from the players.

How do you hide enemy positions in modern or futuristic combat settings without having this interfere with the rules?